I. Field of the Invention
This invention relates to a process for the production of compositions, useful as dielectric fluids, which do not evidence substantial carbon tracking during electrical discharges therein.
II. Description of the Prior Art
During the operation of electrical equipment, such as switches, circuit breakers, transformers, and the like, arcing, sparking or glow discharges usually or occasionally occur, especially at higher voltages. Dielectric materials are commonly employed to reduce or prevent the possibility of such arcing, sparking and glow discharges. For example, solid insulators, such as ceramics or resins, may be used to support or surround electrical conductors. Or, fluid dielectric materials, such as oils or gases, may be used to insulate electrical conductors.
A related problem involves the breakdown of carbonaceous dielectric materials. During arcing, these materials tend to decompose and form carbon, which, being an electrical conductor, not only shortens the gap between conductors, but also eventually leads to carbon bridge short circuits, or carbon tracking. This is a serious problem which has plagued the electrical industry for years.
As used herein, arc interruption includes arc suppression and arc quenching, and refers to preventing or reducing arcing between electrodes. Carbon tracking suppression refers to preventing the formation of carbon during arcing.
Sulfur hexafluoride (SF.sub.6) is well-known as an excellent gaseous dielectric. See, e.g., U.S. Pat. No. 3,059,044, issued to R. E. Friedrich et al., Oct. 16, 1962. It is unique in its electric arc interrupting properties. However, SF.sub.6 does have a few inherent limitations: low vapor pressure at low temperatures, comparatively high freezing point, and relatively high cost.
For some years, it has been known that certain electronegatively substituted carbon compounds (halogenated alkanes) are also highly useful fluid insulators in electrical apparatus. Typical examples are dichlorodifluoromethane (CCl.sub.2 F.sub.2), octafluorocyclobutane (c-C.sub.4 F.sub.8), hexafluoroethane (C.sub.2 F.sub.6), octofluoropropane (C.sub.3 F.sub.8), decafluorobutane (C.sub.4 F.sub.10), trichlorofluoromethane (CCl.sub.3 F), symdichlorotetrafluoroethane (CClF.sub.2 CClF.sub.2), tetrafluoromethane (CF.sub.4), chloropentafluoroethane (CClF.sub.2 CF.sub.3) and chlorotrifluoromethane (CClF.sub.3). While all of the above have reasonably good dielectric strength, it is difficult to prevent spark-over or other electrical discharge from occurring in apparatus containing these materials when high voltage surges develop. The spark-over or other discharge typically leads to carbon track formation.
U.S. Pat. No. 3,650,955, issued to J. A. Manion, et al., Dec. 9, 1966, teaches the use of CCl.sub.2 F.sub.2 combined with c-C.sub.4 F.sub.8 as an arc interrupter gas. However, this combination has been observed to evidence extensive carbon tracking properties.
A mixture of SF.sub.6 and CO.sub.2 has been suggested as a potential gaseous dielectric medium having arc-extinguishing characteristics. See, e.g., U.S. Pat. No. 3,059,044, above. However, no composition range over which such properties may exist is disclosed.
Perhalogenated fluids, including SF.sub.6 and perhalogenated alkanes, have been adsorbed on molecular sieves (zeolites), which are then incorporated as fillers in organic insulators; see U.S. Pat. No. 3,305,656, issued to J. C. Devins, Feb. 21, 1967. During high voltage operation, voids in the insulation are filled by the perhalogenated fluid, which then serves as an arc interrupter. There is, however, no suggestion that such a combination would be useful in carbon tracking suppression.
Attempts have been made to develop gaseous dielectric compositions as carbon tracking suppressants. For example, B. J. Eiseman, U.S. Pat. No. 3,184,533, issued May 18, 1965, teaches the use of an oxygen-containing oxidizing agent, such as SO.sub.2, N.sub.2 O and NO, to suppress carbon tracking of certain electronegatively substituted carbon compounds, such as saturated polyhalohydrocarbon compounds, saturated perhalohydrocarbon compounds, saturated perfluoroethers and the like. However, none of these oxidizing agents is desirable because of their corrosive nature, toxicity, and/or chemical reactivity.
There remains in the art a need for an efficient gaseous dielectric composition that does not evidence carbon tracking.